A pair of small loudspeakers designed to be worn directly on or in the ears. Because these loudspeakers are so close to your ears you should be especially careful with the volume settings. Loud sounds can damage your ears PERMANENTLY!
Each ear piece projects a separate stereo channel (left and right). This separation, and the fact that each loudspeaker is positioned on opposite sides of the head, can result in panning effects sounding much more extreme than they might sound on a pair of loudspeakers (which are usually both positioned in front of you within a standard stereo setting).
Headphones can range in quality and style, with different types being suitable for different purposes. Circumaural headphones cover the whole ear and come in two types. Closed back headphones (best for recording) keep the sound contained around the ear, reducing spillage but potentially inducing some level of phase cancellation as the sounds reflect back off of the inside of the closed back. Open back headphones (best for mixing) allow the sound to spill out into the room, avoiding any phase cancellation, but also allowing other sounds from the local surroundings in.